Temperature compensated shadow mask for a color cathode-ray tube

ABSTRACT

A shadow mask type of three gun color cathode-ray tube, which is otherwise conventional, has flat bi-metallic leaf springs attached at one end to the supporting frame of the mask assembly and provided with apertures at the opposite end to receive studs projecting inwardly from the tube envelope. These leaf springs are dimensioned in relation to the different coefficients of expansion of their bi-metallic components to displace the mask in the direction of the screen as the tube reaches operating temperature in order to avoid misregistration otherwise experienced with thermally uncompensated mask structures.

United States Patent Pappadis 1 Aug. 5, 1975 [54] TEMPERATURE COMPENSATED SHADOW 3.573.527 4/1971 Hafl enscheid et a1. 313/85 S MASK FOR A COLOR CATHODE-RAY TUBE Inventor: Nicholas P. Pappadis, Chicago, Ill.

Assignee: Zenith Radio Corporation, Chicago,

Ill,

Filed: Dec. 19, 1968 Appl. No: 785,171

[52] U.S. Cl 313/405; 313/407 [51] Int. Cl. H01J 29/07; HOIJ 31/20 [58] Field of Search 313/85 S, 92 B, 92 PD [56] References Cited UNITED STATES PATENTS 3,308,327 3/1967 Shrader 313/85 S 3,449,61 1 6/1969 Schwartz et a1. 313/92 B 3,454,813 7/1969 Lewinson 313/85 S X Primary Examiner-Robert Sega] Attorney, Agent, or FirmCornelius J. OConnor; John J. Pederson 5 7 ABSTRACT 2 Claims, 5 Drawing Figures Amsmw TEMPERATURE COMPENSATED SHADOW MASK FOR A COLOR CATHODE-RAY TUBE BACKGROUND OF THE INVENTION The shadow mask of the tube type under consideration is the mechanism for attaining color selection. It has essentially the same size and shape as the image area of the tube and is positioned in close parallel relation therewith. Its function is to confine each electron beam issuing from the gun mount and reaching the screen through the shadow mask to impinge only on those elements of the screen which emit light of the particular color to which that beam and the gun creating it have been assigned. In the usual case the screen is a mosaic of triads of deposits of green, blue and red emitting phosphors and there are three guns in the gun mount one of which excites the green phosphor dots while the remaining two excite only the blue and red dots, respectively, this being accomplished by the aperture pattern of the mask or colorselection electrode. In performing its function, this electrode intercepts a significant portion of each beam and experiences a substantial increase in temperature as the tube achieves operating temperature. It is not uncommon for the mask to rise to a temperature of 80 C.

The structural components of the color-selection electrode will obviously expand in the face of such an increase in temperature ambient. Such expansion, for the case of a thermally uncompensated structure, causes the apertures of the mask to be displaced transversely of the intended beam paths and results in misregistration of the beams with the dots of the phosphor triads. Since the temperature rise can hardly be avoided, it is necessary to take steps to minimize misregistration and a number of proposals have been made to confine movement of the electrode structure along the intended paths of the beams.

For the most part, they have taken the form of sophisticated structural arrangements variously arranged to the end that expansion of the frame component of the color-selection electrode produces axial movement of the mask along the desired beam paths. One prior structure features connecting the conventional leaf springs of the color-selection electrode to the mask frame through a bimetallic coupling structure having one base portion of a low coefficient of expansion, another base portion of a relatively high coefficient of expansion and an expansion loop for interconnecting these base portions. While these structures are more or less successful in reducing misregistration, they are objectionable because they increase the complexity of the tube structure and add significantly to its fabricating costs.

SUMMARY OF THE INVENTION It is an object of the present invention to provide improved and simplified thermal compensation for a color cathode-ray tube which includes a color-selection electrode.

It is a specific object of the invention to simplify the mask assembly of a color cathode-ray tube having thermal compensation.

The invention, as indicated above. is in a color cathode-ray tube having a faceplate section including a rectangular image area bearing deposits of three different phosphor materials defining a multiplicity of phosphor triads, a flange normal to the periphery of the image area, and support pins extending inwardly of the flange on both sides and/or the top and bottom of the tube envelope. The tube further has a color-selection electrode of essentially the same size and shape as the image area and which includes a mask component having a field of apertures individually aligned with an assigned one of the phosphor triads. This electrode further includes a frame normal to the mask component and to which the peripheral portion of that component is attached. Additionally, there is an improved arrangement for mounting the color-selection electrode in spaced parallel relation to the image area of the tube. It comprises a pair of similar elongated leaf springs secured at one end to corresponding points on opposing side portions of the frame and having an aperture at the opposite end dimensioned to accept the support pins of the envelope and form a pivoted connection therewith. The springs face the same direction with respect to the top of the frame and bridge the space between the frame and the pins of the tube. The springs are individually formed of two metallic pieces having different temperature coefficients of expansion, arranged in side-by-side abutting relation with the piece of higher temperature coefficient closer to the image area and mechanically fastened to one another along their abutting surfaces. The length of each spring is proportioned, in relation to the aforesaid difference in the co efficients of expansion, to effect pivotal movement of the springs about their associated pins and axial displacement of the frame toward the image area as the tube attains its operating temperature.

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are be lieved to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIG. I is a schematic representation of a color cathode-ray tube having thermal compensation in accordance with the subject invention;

FIG. 2 is an enlarged fragmentary portion of the tube of FIG. 1 showing with greater particularity details of the mask structure;

FIG. 3 is a sectional view taken on line 3-3 of FIG.

and FIGS. 4 and 5 are enlarged detailed views of the terminal portion of mounting springs which includes its mounting aperture.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more particularly to FIG. I, the cath ode-ray tube there represented is of the well-known shadow mask type and has an envelope l0 terminating at one end in a faceplate section 11. It will be assumed that the tube is of the 23-inch, degree variety having a rectangular image area bearing deposits 12 of different phosphor materials. While the phosphor deposits may be in the form of stripes, it is customary that they be clusters of dots forming the now familiar dot triads and it will be assumed that screen 12 is of this type. In such a case the elements of each triad are deposits of green, blue, and red emitting phosphors and the screen is backed by a conductive and light reflecting layer 13 of aluminum which is sufficiently thin to be electron permeable.

Faceplate section 11 additionally includes a flange 11a that projects from and is substantially normal to the periphery of screen 12 and its internal dimensions exceed the dimensions of the screen. Support pins by means of which the shadow mask is removably installed within faceplate section 11 extend inwardly from flange 11a on both sides and also on the top and/or the bottom of the tube, depending on whether one wishes to employ a three or four-point suspension of the shadow mask. For convenience, a three-point arrangement will be assumed in which case the tube has support pins on both sides of flange 11a and in the central portion of the top. One such pin 14 is shown in the enlarged view of FIG. 3.

The color-selection electrode or parallax barrier 15 of the tube is essentially the same size and shape as screen 12 and has maximum dimensions that are only slightly less than the internal dimensions of tube flange 11a. The color-selection electrode includes a component 16 with electron transparent and electron opaque portions for admitting electrons to selected portions of screen 12. Where the screen is a mosaic of phosphor dots, as has been assumed, this component is the now familiar mask having a field of apertures with each aperture essentially in alignment with one triad of the screen. Obviously, the apertures are electron transparent and the interconnecting mask structure is opaque. The color-selection electrode also has a frame component 17 with a portion 17a extending parallel to the tube axis or perpendicular to mask 16 and to which the peripheral portion of the mask is attached as by welding. The frame further has a portion 17b disposed in normal relation to both the tube axis and to frame portion 170. This assembly of mask 16 and frame 17 is removably installed within faceplate structure 11 by leaf springs to be considered more particularly hereafter. For present purposes they may be considered simply as flexible coupling elements affixed to frame 17 with apertures in their free ends to receive mounting pins 14 extending inwardly of the tube envelope.

At the opposite end of envelope 10 there is the customary gun mount 18 for directing at least one beam along a desired path to screen 12. Usually, this mount is a cluster of three guns each of which issues a beam which, in traversing shadow mask 16, impinges only on phosphor elements of an assigned color as these beams are caused to scan the image area of the tube. Since the tube, as thus far described, is totally conventional both as to its structure and mode of operation, it is not necessary to develop this matter further. Instead, the remainder of the description will be directed to the details of mounting springs 20 and the thermal stabiliza- .tion or compensation they afford.

As stated above, it is assumed that frame 17 is secured to faceplate section 11 by means ofa three-point suspension in which three leaf springs affixed to the mask engage three support pins provided in the envelope. One such spring is positioned essentially in the central top portion of frame 17 with one end welded to the frame and the other end apertured to accept the support pin extending inwardly from the top central portion of the tube envelope. The other two springs are welded to corresponding points on the sides of frame 17 and both face upward. that is to say, the end of each spring closer to the bottom of the frame is welded to the frame and the free end of each spring is closer to or faces the top of the frame. The three springs are structurally alike and one of them is shown in the en' larged fragmentary views of FIGS. 2 to 5.

Each spring is formed of two metallic components 20a and 2012 having specifically different coefficients of expansion. Component 20a, which is closer to screen 12, has a relatively high coefficient of expansion while component 20b, which is closer to gun mount 18, has zero or a relatively low coefficient of expansion. The mounting spring is formed at one end 20d to rest flush against a portion of frame component 17a that is to be positioned in spaced parallel relation to the inner wall of the tube envelope. The other or free end of the spring structure has an aperture 200 that is dimensioned to accept support pin 14 and form a pivoted connection therewith. Thermal stability is best achieved if each of the side springs faces in the same direction, whether they both face the top, as shown, or face the bottom of frame 17. In any event, the specific point of attachment to the frame is chosen to properly present the aperture 200 of each spring to its associated support pin 14. As indicated in FIG. 3, the portion of spring 20 extending from its weld to frame portion 17a is outwardly biased and subtends an acute angle B therewith, being configured to bridge the space between the frame and support pin 14.

With particular reference to FIG. 2, each spring 20 is oriented in relation to screen area 12 to translate the greater expansion of one of its metallic components, experiencd to response in an increase in temperature, to pivoted movement of the spring about its associated pin 14 and axial displacement of frame 17 toward screen 12. More particularly, as the tube is brought to a stable operating temperature, the mask-frame assembly increases in temperature causing its component parts to expand. Expansion of frame 17, for example, tends to compress springs 20, especially those attached to the sides of the frame. Additionally, these springs tend to expand but component 20]; having zero or a low coefficient of expansion resists elongation of the spring and movement of the apertures of mask 16 in a vertical plane. As a consequence of the greater expansion of spring component 20a, the spring tends to bow as indicated by broken-construction line 21 in FIG. 2. Since one end of the spring is held firm, being welded to frame portion 17a, there results a force vector directed generally along line 21, and having a component parallel to the longitudinal axis of the tube but directed away from screen 12. The result of this component acting on the pivoted connection of the mask-frame assembly with support pin 14 is a pivotal movement in the direction of arrow 21a and displacement of mask 16 toward screen 12.

The length of each spring 20 is proportioned, in relation to the difference in coefficients of expansion of its components, to the end that electrons having access to screen 12 through mask 16 impact substantially the same selected portions of the screen at all times, even as the tube, having been energized, attains its operating temperature. That is to say, the proportioning maintains the beams in substantial register with their assigned phosphor dots even though the temperature of the mask structure increases significantly during the first one to two hours of the tube operation. This desired condition is established by constructing springs 20 to cause a particular amount of axial displacement of mask structure per degree rise in temperature to displace the mask assembly essentially along the paths of the electron beams and this is rather easily measured or readily determined empirically.

A preferred structure of the bimetallic mounting spring, and as clearly shown in FIG. 2, is one in which equal lengths of components 200 and 2012 are butt welded to one another. Maximum deflection per degree temperature rise is achieved if only component a, having the higher coefficient of expansion, is welded to mask section 170 as indicated at the points 20e. It is also preferred that the mounting aperture 20c be formed in component 20a. Where the mounting pin 14 is circular in cross section, as represented in FIG. 2, it is preferred to have aperture 200 formed of a first radius r, with cutouts of a radius r as indicated in FIG. 4. And to enhance the pivotal action, it is desirable to countersink or coin the aperture at an angle 0, see FIG. 5.

Illustrative specifications for a bimetallic spring successfully employed in a 23-inch, 90 rectangular tube are as follows:

1. Length of flat section 20d 0.937 inch.

2. Length of remaining section of spring 20 2.375

inches.

3. Angle [3 of spring 20 with frame section 17a 10 Maximum width of component 20a 0.375 inch.

. Maximum width of component 20!: 0.188 inch.

. Radius r 0.ll inch.

. Radius r 0.056 inch.

. Thickness of spring 0.05 inch.

. Composition of component 20a 20% nickel, 3%

chromium, balance iron.

10. Composition of component 20b 36% nickel, maximum 0.1% of carbon, silicon within the range 0.3 to 0.5 percent, and the remainder iron.

Operating such a tube with an anode potential of kilovolts, a beam current of two milliamperes and full raster, resulted in a transverse beam movement with respect to the phosphor dots of l to 1.5 mils as distinguished from a movement of the order of three mils for an equivalent but thermally uncompensated mask asscmbly.

In addition to contributing substantial thermal stability to the assembly, this arrangement maintains the mechanical stability inherent in prior art mounting springs which were of the same structural design but lacking thermal compensation. There is an additional advantage over previous approaches that use a similar bimetallic leaf spring having an extension of the metal component of low coefficient of expansion shaped for attachment to the portion 17b of the frame which is transverse to the tube axis. The attachment to that portion of the frame causes the system to be undesirably susceptible to microphonics which is avoided in the described structure wherein the spring attaches to the portion of the frame that is parallel to the sides of the tube envelope. According to current commercial practices, support pins are affixed to the tube to be received in apertures in the mounting springs but, obviously the reverse arrangement may be employed with the invention, if desired.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true apirit and scope of the invention.

I claim: 1. A color cathode-ray tube having a faceplate section including (a) a rectangular image area bearing deposits of three different phosphor materials defining a multiplicity of phosphor triads, (b) a flange normal to the periphery of said area and (c) support pins extending inwardly from said flange on both sides and on the top and/or bottom of said tube, further having a colorselection electrode of essentially the same size and shape as said image area including (d) a mask component having a field of apertures individually aligned with an assigned one of said triads and (e) a rectangular frame of angular cross section having one part parallel to the axis of the tube to which the peripheral portion of said mask is attached and having another part disposed transverse to said axis, and also having an improved arrangement for mounting said electrode in spaced parallel relation to said image area comprising: a pair of similar elongated leaf springs individually formed of two metallic pieces having different temperature coefficients of expansion, arranged in side-by-side abutting relation and mechanically fastened to one another along their abutting surfaces,

said springs facing the same direction relative to the top of said frame with the piece of higher temperature coefficient closer to said image area,

said springs being secured at one end to said frame by means of weld connections made only to corresponding points on opposing side portions of said one part of said frame, said springs being contoured to bridge the space between said frame and said support pins on the side portions of said faceplate flange and having an aperture at the opposite end dimensioned to accept said support pins and form a pivoted connection therewith,

and the length of each of said springs being proportioned in relation to the aforesaid difference in coefficients of expansion to effect pivotal movement of said springs about their associated pins and axial displacement of said frame toward said image area as the tube attains its operating temperature.

2. The arrangement in accordance with claim 1 in which only that piece of each spring that has the higher coefficient of expansion is welded to said one part of said frame. 

1. A color cathode-ray tube having a faceplate section including (a) a rectangular image area bearing deposits of three different phosphor materials defining a multiplicity of phosphor triads, (b) a flange normal to the periphery of said area and (c) support pins extending inwardly from said flange on both sides and on the top and/or bottom of said tube, further having a color-selection electrode of essentially the same size and shape as said image area including (d) a mask component having a field of apertures individually aligned with an assigned one of said triads and (e) a rectangular frame of angular cross section having one part parallel to the axis of the tube to which the peripheral portion of said mask is attached and having another part disposed transverse to said axis, and also having an improved arrangement for mounting said electrode in spaced parallel relation to said image area comprising: a pair of similar elongated leaf springs individually formed of two metallic pieces having different temperature coefficients of expansion, arranged in side-by-side abutting relation and mechanically fastened to one another along their abutting surfaces, said springs facing the same direction relative to the top of said frame with the piece of higher temperature coefficient closer to said image area, said springs being secured at one end to said frame by means of weld connections made only to corresponding points on opposing side portions of said one part of said frame, said springs being contoured to bridge the space between said frame and said support pins on the side portions of said faceplate flange and having an aperture at the opposite end dimensioned to accept said support pins and form a pivoted connection therewith, and the length of each of said springs being proportioned in relation to the aforesaid difference in coefficients of expansion to effect pivotal movement of said springs about their associated pins and axial displacement of said frame toward said image area as the tube attains its operating temperature.
 2. The arrangement in accordance with claim 1 in which only that piece of each spring that has the higher coefficient of expansion is welded to said one part of said frame. 